Frequency modulation



Nov. 3, 1931. c. w. HANsELI.

FREQUENCY MODULATION Filed March 23. 1928 lllxllll /QCKM'J RNEY R O T N E V m c mm w. Hmseu. BY /Lf Patented Nov. 3, 1931 UNITED STATES CLARENCE W. HANSELL, OF ROCKY IPONT, NEW YORK, ASSIGNOR TO RADIO COPPORA- PATENT' OFFICE TION OF AMERCA, A CORPORATION OF DELAWARE FREQUENCY EOD'ULATION Application filed March 23, 1928. Serial No. 264,102.

This invention relates to frequency modulation, and more particularly to a method and means for obtaining frequency modulation which will accurately follow variations 1n a control current.

With a view to reducing fading, narrowing the necessary frequency spectrum of each communication channel, constantly transinltting maximum energy, and for multiplexing, it has been sug ested to frequency modulate a constant amp itude carrier, rather than to amplitude modulate a constant frequency carrier. It is one object of my invention to provide for varying frequency in accordance l with the magnitude of a modulating or control current, which l do by determining the frequency by resonance in a frequency control circuit including a saturable impedance, and utilizing the control current to vary the saturation of the saturable impedance. A change in the magnitude of the control current changes 'the saturation of the saturable impedance, or magnetic frequency wobbler, and this causes a change in impedance which finally results in changing the natural frequency of the frequency control circuit.

llt is exceedingly desirable, and in fact, 1n the case of multiplexing, essential, that the frequency shift produced'by the magnetic frequency wobbler be in linear relation to the change in magnitude of the control current. For this purpose the range of wobble should be limited, and should be located .at a relatively straight portion of the magnetization curve of the wobbler. This may be done by feeding to the saturation winding of the wobbler a steady current proper in magnitude to cause the desired mean saturation. This requires an absolutely constant supply of current, much more so than is available .from an ordinary power supply line, and t0 irovide a source of constant current in combination-with a frequency Wobbler is a further object of my invention., For this purpose I supply direct. potential from a suitode current is used as the normal saturation current for the wobbler.

Tn the attempt to attain frequency variation which accurately follows the control current I find that an additional source of error is the losses which occur in the magnetic wobbler. One such loss is due to hysteresis, causing the magnetization curve, and consequently the curve showing frequency shift as a function of the control current, to present a hysteretic loop, and this loop is aggravated by the presence of other losses such as eddy currents in the coreof the frequency wobbler, so that the ultimate characteristic is considerably deformed. It is a further object of my invention to overcome this source of error, which I do by applying the control current to the frequency wobbler through an intermediate amplifier, preferably the same electron emission tube as was already em loyed for obtaining constant direct current fJor the normal saturation, and coupling the input and output circuits of the amplifier in proper phase and degree to counteract the effect of4 the losses in the wobbler.

One of the advantages of using frequency modulation is the possibility of reducing the necessary frequency band for each transmitter, but when using a small frequency variation in a high frequency carrier the percentage variation may become so small as to present modulating and demodulating difficulties at the transmitter and receiver. Also, with smallv frequency variations for the signal it becomes especially important to keep the mean frequency constant. This entire problem may be considered from a different aspect, for it is conceded that the mean frequency from the arrangement so far described cannot be kept as constant as modern 5 requirements dictate, and it is desirable to insure constant mean frequency by employing a high frequency oscillator equipped with a crystal drive. Accordingly, my invention'includes using frequency modulated energy of intermediate frequency to modulate the output from a constant frequency oscillator. The lower the mean frequency of the frequency modulated energy is made the less will the mean frequency of the transmittedenergy depend thereon, or vary therewith,

and it is therefore desirable to use a low intermediate frequency. 'At the same time, in order to obtain the desired percentage variation in the frequency of the high frequency carrier it is necessary to provide a relatively large percentage variation in the frequency of the intermediate frequency energy. This enhances the diiiiculty and increases the necessity for maintaining linear response in g5 the frequency modulator, in accordance with the ideas already discussed) My invention is more fully described in the following specification, which is accompanied by drawings in which Figure 1 is a wiring diagram for one form of my invention, and

Figure 2 is explanatory thereof.

Referring to Figure 1 there is an oscillator tube 2, with which is associated regeneratively coupled coils 4 and 6, which form a portion of a frequency control circuit 8, the remainder of which comprises a tuning condenser 10, the radio frequency windings 12 and 14 of a magnetic frequency wobbler 16, and blocking condensers 18 and 20, which shunt radio frequency currents by the sources of direct anode and control electrode potential.

The magnetic frequency wobbler 16 comprises two similar cores 28 and 30, each of which is composed of a stack of thin laminations in the form of annular disks. The radio frequency windings 12 and 14 are wound around an annular portion of the cores, that is, they are threaded through the center and around the outside of each of the cores, so that magnetic flux flows around the annular rings. The coils 12 and 14 are connected in series with each other and with the tuned frequency controlling circuit 8. The control current is applied to the saturation windings 32 and 34, which are wound like the radio frequency windings, and which also are connected in series, but which are connected in opposite sense to the radio frequency windings, so that radio frequency ener induced in them is neutralized, the coils eing connected relatively in opposition.

Attention is new directed to Figure 2, in which the frequency shift in the circuit 8 is plotted asa function of the control current applied to the saturation winding, and it will be seen from the mean curve 100, that for suitable linear response only a limited portion, such as that between the vertical lines 102 and 104, should be chosen for the operating range. This requires a normal or steady control current having the value indicated bv the line 106, and to provide such a constant control current, despite fluctuations in the potential of the power source, I employ an arrangement which is further described in connection with Figure 1.

In that figure it will be seen that the steady saturation current for the saturation windings 32 and 34 is obtained from the anode circuit of an electron emission tube 36, energized by connection at points 38 and 40 to a potentiometer resistance 42, which is connected across a direct current line 44, and optionally through an intermediate control rheostat 46. The control electrode of the tube 36 is connected, as shown, to the potentiometer 42 at a point 48 which is more negative than the point 38 to which the filament is connected. By this arrangement fiuctuations in the potential of the line 44 cause opposite potential variations on the control electrode and anode of the tube 36, the magnitudes being proportional to the spacing of the points 40, 38, and 48 38, on the resistance 42. For example, on increased potential there is a tendency towards increased anode current owing to the anode being made more highly positive, but the increased negative potential of the control electrode tends to diminish the anode current and keep it constant. By arranging the contacts 38, 40 and 48 at proper points, the ratio of spacing of which will approximate the amplification factor of the tube, the anode current may be kept perfectly constant. Since the point 48 will not, in general, provide the correct grid bias when properly located for current correction, a C battery 50 may be provided, in order to obtain the desired anode current. This battery will have long life at constant voltage because there is no grid current fiow.

Again referring to F igure 2 it will be seen, by the dotted lines 108 and 110, that the losses in the frequency wobbler, more particularly the hysteresis and eddy current losses, deform the characteristic of the wobbler, and so prevent accurate linear response. When the limited range of wobble is used the curve becomes that indicated at 112. I avoid this by the simple expedient of regeneratively coupling the input and output circuits of the tube 36, as is indicated by the transformer 52 in Figure l. This coupling is preferably made variable, so that the amount of output potential fed back to the input circuit may be adjusted to just balance out the effect of the losses, thereby making the curves 108 and 110 in Figure 2 coalesce to form the mean curve 100.

The primary control or modulating current may be superimposed on the normal saturation current by connection at any suitable point in the circuits described, but preferably is coupled by means of a suitable transformer 54 to the input circuit of the vacuum tube 36. The control or modulation current may be speech, from a microphone 56 in series with a direct current source, or code signalling by means of alternating current from a generator 57 controlled by a key 58. A plurality of such alternating current circuits of differing frequencies may be used to multiplex, as is more fully disclosed in my copending application Serial Number 212,192, filed Aug. 11, 1927.

The frequency modulated energy may be taken from either or both of the coils 4 and 6, and after suitable amplification, directly used for transmission. To insure constant mean frequency I prefer, especiallyin the case of short Wave communicatiom'to utilize a high frequency oscillator especially designed for the production of energy of constant frequency, and to modulate this energy with frequency modulated energy of intermediate frequency. Thus, in Figure 1, I tune the circuit 8 to a relatively low intermediate frequency, and l employ a high frequency oscillator 60, provided with a crystal drive 62, the output from which is fed through radio frequency condensers 64 to the carrier suppression modulator tubes 66 in parallel. The intermediate .frequency energy is fed from the coils 4 and 6 to the coil 66, which is coupled to the modulator tubes 66 in series. The output coil 70 is connected to the tube anodes in series also, in consequence of which the carrier components oppose in the halves of the coil 70 and are neutralized. @f the side bands the filter 72 selects one, and this is amplified in a power amplier T4. rllo insure constant energy output a limiter may be used, and preferably the power amplifier 74 is itself made to act as a limiter, by increasing the amplitude of the excitation on cach stage beyond the point Where an increase in excitation causes appreciable increase in output. This incidentally is the adjustment which is desirable to secure high energy efficiency in an amplifier which is to be operated at constant output or with keyed modulation. rlThe amplifier output is radiated in a suitable antenna circuit 7 6.

l claim:

1. The method of causing the frequency variations obtained from a magnetic frequency wobbler. the normal saturation current for which is fed from a direct current source, to accurately follovi7 variations inthe control current despite variations in the potential of the direct current source, which includes applying potential from the source to the anode circuit of an electron emission tube, applying potential from the same source to the control electrode of the tube in opposite sense and proper proportion to counteract the effect of the potential fluctuations of the source, and using the resulting constant anode current as the normal saturation current for the magnetic frequency wobbler.

2. The method of causing the frequency variations obtained from a magnetic frequency wobbler to accurately follow variations in the control current despite losses in the wobbler which includes applying the control current to the wobbler through an intermediate amplifier, and feeding a portion of the output energy of the amplifier to the input thereof in proper phase and degree to counteract the effect of losses in the wobbler.

3. The method'of causing the frequency variations obtained from a magnetic frequency wobbler to accurately follow7 variations in the control current which includes applying potential from a direct current powersource to the anode circuit of a vacuum tube, applying potential from the same source to the control electrode of the tube in opposite sense and proper proportion, to counteract the effect of potential fluctuations of the source, using the resulting constant anode current' as a normal saturation current for the wobbler, applying the wobble control current to one of the circuits of the tube, and' feeding a portion of the anode circuit energy to the control electrode circuit of the tube in proper phase and degree to counteract the effect of losses in the wobbler.

4. A frequency modulator comprising a tuned frequency controlling circuit, a saturable impedance in said tuned circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof, a source of control current coupled to the saturation Winding, and means to apply thereto an additional current variation of proper magnitude'and phase t0 counteract the effect of losses in the saturable impedance.

5. A frequency modulator comprising a tuned frequency controlling circuit, a saturable impedance in said tuned. circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof. a source of constant cunrent coupled to said Winding to establish a desired normal saturation, a source of control ,current coupled to the saturation Winding,

and means to apply thereto an additional current variation of proper magnitude and phase to counteract the effect of losses in the saturable impedance.

6. A frequency modulator comprising a tuned frequency controlling circuit, a saturable impedance in said tuned circuit, a saturation Winding associated with the core of the saturable impedance for controlling saturation thereof, a direct current source, a resistance connected thereto, an electron emission tube having its anode and control electrode circuits so coupled to the resistance that the normal anode current remains constant despite variations in the potential of the source, means connecting the saturation Winding in series with the anode circuit of the tube, and a source of control current coupled to the input circuit of the tube.

7. A frequency modulator comprising a tuned fre uency controlling circuit, a saturable impe ance in said tuned circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof, an electron emission tube having anode and control electrode circuits, means coupling the saturation Winding to the anode circuit, a source of control current, means coupling the source to the control electrode circuit, and means regeneratively coupling the control electrode and anode circuits to compensate for losses in the Wobbler.

8. A frequency modulator comprising a tuned frequency controlling circuit, a saturable impedance in said tuned circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof, an electron emission tube having anode and control electrode circuits, means coupling the saturation winding to the anode circuit, a source of direct current, a resistance connected thereto, means so coupling the anode and control electrode circuits to said resistance that the normal anode current remains constant despite variations in the I tential of the source, a source of control current coupled to the control electrode circuit of the tube, and mea-ns regeneratively coupling the control electrode and anode circuits of the tube to compensate for losses in the Wobbler.

9. A frequency modulation transmitter comprising an intermediate frequency oscillator having a tuned frequency controlling circuit, a saturable impedance in said turned circuit, a saturation Winding 'associated with the core of the saturable impedance for controlling the saturation thereof, a source of modulation current coupled to the saturation Winding, a source of constant high frequency energy, means for combining the intermediate and high frequency energies to produce side band energy, and means for radiating a portion of the side band energy so produced.

10. A frequency modulation transmitter comprising an intermediate frequency oscillator having a tuned frequency controllin circuit, a saturable impedance 1n said tune circuit, a saturation Winding associated with the core of the saturable impedance for controlling` the saturation thereof, a direct current source, a resistance connected thereto, an electrony emission tube having its anode and `control electrode circuits so cou led to the resistance that the normal anode current remains constant despite variations in the potential of the source, means connecting the saturation winding in series with the anode circuit of the tube, a source of modulation current coupled to one of the circuits of the tube, a source of constant high frequency energy, means for combining the high frequency energy and intermediate frequency energy from the intermediate frequency oscillator to produce side band energy, means for amplifying a portion of the side band energy, and means for radiating the amplified side band energy.

11. A frequency modulation transmitter comprising an intermediate frequency oscillator having a tuned frequency controlling circuit, a saturable impedance in said tuned circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof` an electron emission tube having anode and control electrode circuits, means coupling the saturation-Winding to the anode circuit, a source of direct current, a resistance connected thereto, means so coupling the anode and control electrode circuits to said resistance that the normal anode current remains constant despite variations in the potential of the source, a source of modulation current coupled to the control electrode circuit of the tube, means regeneratively coupling the coutrol electrode and anode circuits of the tube to compensate for losses in the Wobbler, a source of constant high frequency energy, means for combining the high frequency energy and intermediate frequency energy from the intermediate frequency oscillator to produce side band energy, means for amplifying a portion of the side band ener and means for transmitting the ampli ed side band energy.

l2. A frequency modulation transmitter comprising an intermediate frequency oscillator having 'a frequency controlling circuit, a saturable impedance in said frequency controlling circuit, a saturation Winding associated with the core of the saturable impedance for controlling the saturation thereof, a source of modulation energy coupled to the saturation Winding, a source of constant high frequency energy, means for combining the high frequency energy with energy from the frequency controlling circuit, and means for transmitting some of the combined energy.

13. A frequency modulation transmitter comprising an intermediate frequency oscillator having a frequency controlling circuit, a saturable impedance in said controlling circuit, a saturation Winding associated With the core of the saturable impedanceifor controlling the saturation thereof, a diiect current source, an impedance connected thereto, an electron emission tube having its anode and control electrode circuits so coupled to said last mentioned impedance that the normal anode current remains constant despite variations in the potential of the source, means coupling the saturation winding with the anode circuit of the tube, a source of modulation energy coupled to one of the circuits of the tube, a source of constant high frequency energy, means for combining ener from the high frequency source and rom the fre uency controllin circuit to produce side (iiand ener and means for transmitting a portion of t e side band energy so roduced.

CLAPENCE W. HANSELL. 

